1. Field of the Invention
This invention concerns a magnetic thin film capable of recording data at high density for use in storage media such as video tapes, magnetic discs and optomagnetic discs, and a method of manufacturing the same.
2. Description of the Prior Art
Magnetic recording has been directed to higher density and digital recording in recent years. Magnetic recording method mainly employed so far has been the so-called interplaner magnetization method in which directions of easy magnetization exist on a plane of a magnetic recording medium. However, since the directions of magnetization in the magnetic recording medium are oriented so as to repell with each other as the recording density is increased in this system, it has been difficult to increase the recording density. In view of the above, a magnetic recording method called perpendicular magnetization, in which directions of easy magnetization are in the direction perpendicular to the plane of the magnetic recording medium, has been recently developed as a new magnetic recording method, which has enabled the art to outstandingly increase the recording density as disclosed in "An Analysis for the Magnetization Mode for High Density Magnetic Recording", by S. Iwasaki and Y. Nakamura, IEEE Transaction, Magn. MAG-13, No. 5, p 1272 (1977). A cobalt-chromium (Co-Cr) alloy film has been developed as the perpendicular magnetic recording recording medium mainly by a sputtering process as disclosed in "Co-Cr Recording Film with Perpendicular Magnetic Anisotropy", by S. Iwasaki and K. Ouchi, IEEE Transactions, Magn. MAG-14, 5, 849 (1978). In addition to the Co-Cr alloy, barium ferrite (BaO.6Fe.sub.2 O.sub.3) has been obtained by the sputtering process as disclosed in "Structure and Magnetic Properties of C-Axis Well Oriented Ba-Ferrite Films Deposited by Targets-Facing Type of Sputtering", by Hoshi, Matsuoka, Naoe and Yamanaka, The Transactions of the Institute of Electronics and Communication Engineers of Japan (C), J. 66-C, 1, p 9-16 (January, 1983).
In these perpendicular magnetic recording media, although the Co-Cr alloy film can be prepared at a low temperature, the perpendicular magnetic anisotropy thereof as the measure for the magnitude of the perpendicular magnetization is smaller than that of barium ferrite and strontium ferrite. This causes a problem that no complete perpendicular magnetization film can be obtained, but some inplanar magnetized components remain. Further, since the Co-Cr alloy is a metal material, it is readily oxidized as other magnetic materials such as Fe and Co-Ni. While on the other hand, a substantially complete perpendicular magnetization film can be prepared with barium ferrite and strontium ferrite since a substantially complete C-axis oriented film can be obtained therewith. However, since the substrate temperature has to be 500.degree. C. or higher for preparing a barium ferrite or strontium ferrite film, it is difficult to prepare strontium ferrite or barium ferrite on a polyimide or aluminum substrate.
While on the other hand, the perpendicular magnetization is necessary also in the optothermal magnetic recording for attaining high density recording.
However, the optothermal magnetic recording method is different from the magnetic recording method in that the change in the magnetic property due to heat application is utilized for recording and the optical effect relevant to the magnetic property is utilized for reproducing. That is, heat of a laser beam is utilized for recording and Kerr effect or Faraday's effect of the optomagnetic recording medium is utilized for reproduction as disclosed by Osamu Imamura, The Journal of the Institute of Television Engineers of Japan, Vol. 39, No. 4 (1985), p 365-368. Further, if an optomagnetic disc is used, for example, as the medium, a large Kerr effect (large Kerr rotation angle) is required in order to improve the SN ratio (signal to noise ratio) of the disc as disclosed in "A Guide To Getting Strong Magneto Optical Effect", Masanori Abe, Journal of Magnetics Society of Japan, Vol. 8, No. 5 (1984), p 366-372.
In view of the above, optomagnetic recording media with large Kerr rotation angle such as manganese-bismuth (MnBi), gadolinium-cobalt (GdCo) and gadolinium-terbium-iron (GdTbFe) have been developed by the vacuum deposition or sputtering process.
However, since these recording media utilize metal thin films such as Gd, Tb and Fe which are readily oxidized, they are considered not adaptable to external memory devices or the like for computers that require high reliability.
While on the other hand, it has been attempted to use ferromagnetic oxides, which are highly stable chemically, as the optomagnetic or magnetic recording (medium as disclosed by) J. W. D. Martens and A. B. Voermans, IEEE Transactions on Magnetics, vol. MAG-20 No. 5, September, 1984, and mainly cobalt ferrite films or oxide iron films are prepared by a heat treatment at 400.degree. C.-800.degree. C. using a sputtering or gas phase heat decomposing process (by H. Schmid in Austrian Pat. No. 162,382 (1949), and R. H. Sawyer in U.S. Pat. No. 2,642,339 (1953). In these optothermo magnetic recording media, although MnBi, GdCo and GdTbFe alloys can produce perpendicular magnetization films through synthesis at lower temperature, there has been a problem that the reliability is reduced due to the oxidation of the film. Particularly, in the case of using inexpensive substrates such as of polycarbonate or polyimide, these substrates are liable to adsorb water and, accordingly, the alloys may be oxidized due to the absorbed water.
While on the other hand, ferrites such as cobalt ferrite are free from oxidation of the film and, stable and inexpensive. However, since a heat treatment at 700.degree. C.-800.degree. C. (crystallization) is necessary upon preparing the film through sputtering or chemical vapor deposition process (CVD process) for obtaining a film of a large Kerr effect, it is difficult to use those substrates, for example, made of low melting glass, aluminum, polycarbonate and polyimide. In addition, since the ferrites such as cobalt ferrites have a spinel type isotropic crystal structure, there is a problem that they cannot form a perpendicular magnetization film by the crystal magnetic anisotropy such as in barium ferrite or Co-Cr.